(1) Field of the Invention
The present invention relates to a solid-state imaging element used in a digital camera or the like and to a solid-state imaging device.
(2) Description of the Related Art
With the widespread proliferation of digital cameras and mobile telephones with cameras in recent years, the market for solid-state imaging elements has markedly grown. Presently, in CCDs and CMOS image sensors widely used as solid-state imaging elements, semiconductor integrated circuits having plural light receiving units are two-dimensionally arrayed to convert an optical signal from an object into an electrical signal.
As a demand with respect to solid-state imaging elements, there is a demand for increasing the sensitivity as well as increasing the number of pixels and the resolution. The sensitivity of a solid-state imaging element is determined by the magnitude of an output current from a light receiving element with respect to the quantity of incident light. Therefore, introduction of incident light into the light receiving element with reliability is an important factor in improving the sensitivity.
FIG. 5 is a diagram showing an example of the structure of a conventional ordinary solid-state imaging element (pixel). Incident light 61 which perpendicularly enters a microlens 62 of a solid-state imaging element 200 is separated by a color filter 2 of red (R), green (G) or blue (B) and thereafter converted into an electrical signal by a light receiving element 6 (Si photodiode). Microlenses are used in almost all solid-state imaging elements, because relatively high light collection efficiency can be obtained by using the microlens. The solid-state imaging element 200 further has Al wiring conductors (light shielding films) 3, signal transmitting units 4 and planarizing layers 5, as shown in FIG. 5.
Solid-state imaging elements presently have an extremely fine structure such that the pixel size (also referred to as “cell size”) is 2.2 μm. However, a much smaller pixel size is required for a further improvement in resolution in future. A microlens is processed on the submicron order to have such a pixel size and the microlens cannot be formed by heat reflow in the current process. Therefore, the development of a novel fine optical element with which microlenses will be replaced is indispensable to realize further improvements in sensitivity and resolution of solid-state imaging elements in future.
With the development of the planar process techniques typified by optical lithography and electron beam lithography in recent years, a light collecting element having a structure with a periodicity in a subwavelength region (subwavelength lens: SWLL) has attracted attention. “Subwavelength region” is referred to as a region of wavelengths substantially equal to or shorter than the wavelength of light to be collected. A research group at the University of Delaware has demonstrated by simulation that when a Fresnel lens which is an aspherical lens is changed into an SWLL in lattice form, it has a light collecting effect (see, for example, “D. W. Prather, Opt. Eng. 38 870-878 (1999)”). This SWLL is formed by a method of dividing a conventional Fresnel lens (FIG. 1(a)) by the period (width: d) of a λ/2n (λ: the wavelength of incident light, n: the refractive index of the lens material) region 63 and performing linear approximation (FIG. 1(b) and approximation to the rectangular shape (FIG. 1(c) in each region. It has also been reported that a blazed binary optical diffraction element was formed by controlling the line width of the structure in a sub-wavelength region, and that the diffraction efficiency was thereby improved (see, for example, Japanese Unexamined Patent Application Publication No. 2004-20957).
If a SWLL can be used as a light collecting element for a solid-state imaging element, a microlens can be formed by the ordinary semiconductor process and the shape of the lens can be freely controlled.
FIG. 2 shows a basic structure of a solid-state imaging element incorporating a SWLL-type light collecting element 1. In FIG. 2 is illustrated a state in which a SWLL having a submicron fine projection/recess structure is on-chip-mounted in place of a microlens. The film thickness (height) of the light collecting element 1 is 0.5 μm.
FIG. 3 is a top view of the SWLL-type light collecting element 1. The concentric circle structure in the light collecting element 1 is formed of a high-refractive-index material 65 (TiO2 (n=2.53)) and a low-refractive-index material 66 (air (n=1.0)), and the period 63 between adjacent circular light-transmitting films is 0.2 μm.
The line width in the concentric circle structure of the light collecting element 1 has the maximum value at a central portion of the circle and inner to outer rings are successively reduced in line width. When the period is substantially equal to or shorter than the wavelength of incident light, the effective refractive index that affects light is determined by the ratio of the volumes of the high-refractive-index material and the low-refractive-index material. A lens having this structure is a distributed index lens in which the effective refractive index is reduced along a direction from the center of concentric circles to the outer periphery. The division period (e.g., the region 63 in FIGS. 1A to 1C) of this SWLL depends strongly on the wavelength of target incident light and is, therefore, about 0.1 to 0.3 μm in the visible light region.
The above-described conventional method requires making the structure finer (0.01 to 0.1 μm) in this region. However, the period of even the finest structure obtained by the current process techniques is limited to about 0.07 μm.
FIG. 4 shows a light collecting profile of the SWLL. The direction in which incident light travels corresponds to the direction from the bottom to the top of FIG. 4. The incident light perpendicularly enters the lens. A light component 60 scattered at the lens surface can be recognized as well as light component 59 which is being effectively collected. This is due to the fact that an abrupt change in refractive index cannot be realized because of the large structure of the light collecting element 1.